\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\begin{document}
\small
\begin{tikzpicture}[>=stealth,scale=1.2]
  \draw [thin,<->](0,2.5)node[above]{$U$}--(0,0)node[below]{$O$}--(3.5,0)node[above]{$I$};
  \draw [thick,dashed] (0,2)node[left]{$\mathcal{E}$}--(3,2); 
  \draw [thick](0,2)--(3,1.5);   
\end{tikzpicture}
\end{document}